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A175355 Noncomposite reverse concatenations of divisors of n, sorted by n. 3
1, 31, 421, 71, 131, 191, 2551, 311, 391331, 431, 48241612864321, 911371, 971, 1031, 1091, 1173913931, 1511, 1571, 1811, 1931, 2111, 2411, 24412261421, 24719131, 2711, 289171, 29214673421, 3014371, 30910331, 32565251351, 3371, 3491, 3671 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A176558(A089374(n)).

a(n) = A069582(n-1), n>1. [R. J. Mathar, May 03 2010]

EXAMPLE

For n = 9; a(9) = 391331 because A089374(9) = 39 and divisors of 39 are 1, 3, 13, 39; reverse concatenation of divisors A176558(21) = 391331 is noncomposite.

MAPLE

rcd:= proc(n) local D, T, i;

  D:= sort(convert(numtheory:-divisors(n), list));

  T:= D[1];

  for i from 2 to nops(D) do

    T:= T + 10^(1+ilog10(T))*D[i]

  od;

  T

end proc:

select(t -> t=1 or isprime(t), map(rcd, [$1..1000])); # Robert Israel, Aug 12 2020

CROSSREFS

Subsequence of A176558(n).  Supersequence of A055781.

Sequence in context: A268137 A089375 A069582 * A068658 A068672 A274202

Adjacent sequences:  A175352 A175353 A175354 * A175356 A175357 A175358

KEYWORD

nonn,base

AUTHOR

Jaroslav Krizek, Apr 20 2010

EXTENSIONS

Edited by Charles R Greathouse IV, Apr 30 2010

STATUS

approved

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Last modified March 5 01:03 EST 2021. Contains 341812 sequences. (Running on oeis4.)